1. Field of the Invention
The present invention relates generally to communication systems. More specifically, a multiplexed CODEC for an ADSL central office is disclosed that processes signals sent and received on multiple data lines. In addition, a multiplexed CODEC is disclosed for an ADSL remote unit that generates a data signal for sending an upstream signal to the central office as well as an echo cancellation signal that is used to process an incoming downstream signal from the central office. A multiplexed CODEC at the ADSL remote unit also is disclosed that may be used to process both ADSL signals and voice band modem POTS signals.
2. Description of the Related Art
Asymmetric Digital Subscriber Line (ADSL) technology is used to deliver high-rate digital data over existing phone lines without modification of the phone lines. A modulation scheme called Discrete Multitone (DMT) allows the transmission of high speed digital data. ADSL facilitates the simultaneous use of normal telephone services, ISDN, and high speed data transmission. ADSL allows a large bandwidth (1.104 MHz) for transmission in a downstream direction from a central office to a subscriber in a remote location and a lesser bandwidth (138 kHz) for transmission in an upstream direction from a subscriber in a remote location to a central office. ADSL systems are described further in the ADSL T1E1.413 standard, which is herein incorporated by reference for all purposes.
FIG. 1A illustrates the architecture of an ADSL system. A digital network 100 provides data that is to be sent downstream over the ADSL line. The data is delivered to an ADSL transceiver unit in a central office (an ATU-C) 102. The ATU-C modulates data for downstream transmission using a DMT scheme. ATU-C drives a line 104 with an analog signal. Line 104 is connected to a telephone line 110 that is part of a Public Switched Telephone Network (PSTN) 106 by a splitter 108. ATU-C also receives data transmitted in the upstream direction on line 104.
At the receiver end, a splitter 120 splits the signal on line 110 between a message telecommunication service (MTS) 122 (also referred to as plain old telephone service (POTS)) and an ADSL transceiver unit line for a remote unit (an ATU-R) 124. ATU-R 124 is connected to one or more service modules (SM) 128 by a bus 126. The ATU-R demodulates data transmitted by the ATU-C in the downstream direction and also transmits data in the upstream direction.
FIG. 1B is a block diagram illustrating the transmission path of an ADSL system. The transmission path may be in either the upstream or the downstream direction. An encoder 150 sends digital data to a DAC 152. The analog output of DAC 152 represents an ADSL modulated signal that is to be transmitted on an ADSL line. The analog output of DAC 152 is therefore directed to a reconstruction filter 154. Reconstruction filter 154 reconstructs the signal that is to drive an ADSL line 156. Usually, there is an additional line driver in the signal path. Thus, the signal that drives ADSL line 156 is first generated digitally by encoder 150. DAC 152 converts the digital signal to an analog signal and reconstruction filter 154 drives the ADSL line.
FIG. 1C is a block diagram illustrating an asymmetric digital subscriber line (ADSL) system that includes a central office 160 and several remote terminals 162. Typically, the central office includes a large number of lines extending to a large number of remote terminals. For economic reasons, it is desirable that the central office be able to accommodate as many lines as possible. A number of factors limit how many lines may be supported in a central office building. A major factor is the ability of the building to dissipate sufficient heat so that the heat generated by the ADSL transceiver and modulation hardware does not cause systems to fail. Therefore, it is important to design systems so that the power dissipated per line to a remote terminal is minimized, making it possible to maximize the number of lines included in a central office building.
One approach to increasing the number of lines that can be included in a central office is to design ADSL central office chip sets with multiple line architectures. Including circuitry for more than one line on a chip may have the advantage of reducing the amount of heat generated for each line in the central office as well as reducing manufacturing costs.
FIG. 2 is a block diagram illustrating a prior art system that includes a multiple line ADSL transceiver that generates downstream transmitted ADSL signals on two ADSL lines. An ADSL transceiver 200 transmits and receives ADSL signals to and from a CODEC 202. In the downstream direction, CODEC 202 converts a digital signal from the ADSL transceiver to an analog signal and sends the analog signal to a line driver 204. Line driver 204 generates an analog modulation signal that can be applied to the first ADSL line supported by the ADSL transceiver. The output from the line driver is input to a hybrid 206. Hybrid 206 is connected to the ADSL line usually through a transformer.
Incoming upstream signals from the ADSL line are transferred by hybrid 206 to a line receiver driver 205. The analog signals from line receiver 205 in the upstream direction are converted to digital signals by CODEC 202 and those digital signals are input to the ADSL transceiver 200. In the same manner, for the second line supported by ADSL transceiver 200, digital signals from the ADSL transceiver are converted to analog signals in the downstream direction by a CODEC 212. Analog signals from CODEC 212 are input to line driver 214 which drives the second ADSL line through hybrid 216. Also, incoming upstream signals from ADSL line 2 are input to the hybrid 216 which transfers the signals to line receiver 215 so that they may be converted to digital signals by CODEC 212. The digitized signals from CODEC 212 are input to the second ADSL port of ADSL transceiver 200.
The benefit of this architecture is that a single ADSL transceiver may be used for two lines instead of requiring an ADSL transceiver for each line. It would also be useful if the number of other chips in the chip set could be reduced as well. For example, one line driver and CODEC is required for each ADSL line. If a system could be designed that would allow such chips to support more than one ADSL line, then further power and cost saving could be realized.
Accordingly, a system and method for sending and receiving ADSL signals for more than one ADSL line at an ATU-C is disclosed that requires only a single CODEC for multiple lines. Signals output from an ADSL transceiver are time division multiplexed and input into a single CODEC. The output of the CODEC is switched and sent to different line drivers for each of the lines associated with the transceiver. Similarly, in the upstream direction, multiple lines are connected through receivers to the CODEC and the CODEC samples each of the lines alternatively and interleaves the digitized data on a signal digital output to the transceiver. In addition, a system and method implemented at an ATU-R is disclosed that also includes a multiplexed CODEC. The multiplexed CODEC is used to receive an ADSL transmission and also process an echo canceling signal in some embodiments. The multiplexed CODEC is also used to process a modem signal from a POTS line in some embodiments.
It should be appreciated that the present invention can be implemented in numerous ways, including as a process, an apparatus, a system, a device, a method, or a computer readable medium. Several inventive embodiments of the present invention are described below.
In one embodiment, an ADSL central office transmission system for transmitting downstream DMT signals to a plurality of remote ADSL transceiver is disclosed. The system includes a DMT digital signal transceiver that generates a time division multiplexed digital signal that includes a plurality of DMT signals to be sent on a plurality of ADSL lines. A digital to analog converter converts the time division multiplexed digital signal into a time division multiplexed analog signal that includes a plurality of analog DMT signals. The digital to analog converter has an output that outputs the time division multiplexed analog signal. A switch selectively connects the output of the digital to analog converter to each of a plurality of transmitters. The transmitters are configured to drive the plurality of ADSL lines. Thus, the plurality ADSL lines are driven by the plurality of analog DMT signals.
In another embodiment, a method of transmitting a plurality of downstream DMT signals from a central office is disclosed. The method includes generating a time division multiplexed digital signal that includes a plurality of digital DMT signals. The time division multiplexed digital signal is converted into a time division multiplexed analog signal that includes a plurality of analog DMT signals. The time division multiplexed analog signal is selectively connected to each of a plurality of transmitters and a plurality of ADSL lines are driven with the plurality of transmitters.
In another embodiment, an ADSL remote transceiver for receiving DMT signals from a central office and for transmitting DMT signals to the central office is disclosed. A DMT digital signal encoder generates a digital DMT signal to be sent to the central office. A digital to analog converter inputs the digital DMT signal to be sent to the central office. The digital to analog converter has a digital to analog converter output that outputs an analog transmission signal for transmission to the central office. An analog echo cancellation signal cancels echoes caused by leakage from the analog transmission signal in a received signal from the central office. Thus, a transmission signal is generated for sending data to the central office and an echo cancellation signal is generated for correcting errors in the received signal.
In another embodiment, a method of receiving DMT signals from a central office and for transmitting DMT signals to the central office is disclosed. The method includes generating a digital DMT signal to be sent to the central office. A digital DMT signal to be sent to the central office is converted to an analog DMT transmission signal and to an analog echo cancellation signal for correcting errors caused by leakage from the analog DMT transmission signal in a received signal from the central office. A time division multiplexed analog output is generated that includes the analog DMT transmission signal and the analog echo cancellation signal. The time division multiplexed analog output is selectively connecting to an ADSL transmission line and to an echo cancellation circuit.
In another embodiment, a remote unit is configured to transmit both an ADSL signal and a POTS modulated signal. The remote unit includes a DAC having an input and an output and a DMT encoder selectively connected to the input of the DAC. A POTS modem encoder is selectively connected to the input of the DAC in a complementary manner such that when the DMT encoder is connected to input of the DAC, the POTS modem encoder is not connected to the input of the DAC and when the POTS modem encoder is connected to the input of the DAC the DMT encoder is not connected to the input of the DAC. An ADSL transmitter and a POTS modem are selectively connected to the output of the DAC so that when the DMT encoder is connected to the input of the DAC, the ADSL transmitter is connected to the output of the DAC and when the POTS modem encoder is connected to the input of the DAC, the POTS modem is connected to the output of the DAC.
In another embodiment, a remote unit is configured to receive both a DMT signal from an ADSL central office and a modulated POTS signal. The remote unit includes an ADC having an input and an output and an ADSL line selectively connected to the input of the ADC. A POTS line is selectively connected to the input of the ADC in a complementary manner such that when the ADSL line is connected to input of the ADC, the POTS line is not connected to the input of the DAC and when the POTS line is connected to the input of the ADC, the ADSL line is not connected to the input of the ADC. An ADSL processor and a modem processor selectively connected to the output of the ADC so that when the ADSL line is connected to the input of the ADC, the ADSL processor is connected to the output of the ADC and when the POTS line is connected to the input of the ADC, the modem processor is connected to the output of the DAC.
These and other features and advantages of the present invention will be presented in more detail in the following specification of the invention and the accompanying figures which illustrate by way of example the principles of the invention.